1. Technical Field
The present invention relates to a propylene-based polymer, to a polypropylene resin composition containing the polymer and to an injection-molded article comprising the composition. In particular, the present invention relates to a polypropylene resin composition having a high die swell ratio such that when being injection-molded, it is difficult for flow marks to occur. The resulting molded article has a good weld appearance, and it is difficult for sinking (facial strain) to occur in the surface of the molded article. A good injection filling property is achieved, and hard spots are inhibited to occur in the molded article. The present invention relates also to a propylene-based polymer suitable as a component of the above-mentioned polypropylene resin composition and to an injection molded article made of the composition.
2. Description of the Related Art
Polypropylene resin compositions are used for a wide variety of applications such as molded articles, e.g. automotive interior or exterior materials and electric appliance housings because they are materials excellent in rigidity, impact resistance and the like.
For example, JP-A-2002-194023, which is published also as U.S. 2004/0014871 A1, discloses a random propylene polymer capable of controlling the appearance and the moldability of a polypropylene resin composition with a good balance by compounding a small amount of propylene-ethylene block copolymer, a random propylene polymer having a die swell ratio of from 1.5 to 2.5, a ratio of the weight average molecular weight Mw to the number average molecular weight Mn (i.e. Q value: Mw/Mn) determined by gel permeation chromatography (GPC) of from 7 to 13. Specifically, JP-A-2002-194023 discloses a random propylene polymer obtained by two-stage polymerization comprising producing, in a first state, 70% by weight of a propylene homopolymer having an MFR of 150 g/10 min and then producing supplying a mixed gas of propylene and ethylene (ethylene content=6% by weight) in the presence of the polymer formed in during the first stage to produce 30% by weight portion of the whole polymer. Moreover, a polypropylene resin composition containing the random propylene polymer, and an injection-molded article made of the resin composition are also disclosed.
However, polypropylene resin compositions to be used for injection-molded articles have been required for further improvement, for example, controlling the occurrence of flow marks, in other words, increasing the die swell, improving the weld appearance, controlling the occurrence of sinking in the surface of a molded article to improve the injection filling property, and inhibiting the occurrence of hard spots. For such improvement, there is a demand for propylene-based polymer available for forming polypropylene resin compositions to be used for the production of injection-molded articles.